A process is known from U.S. Pat. No. 6,191,251 (R. U. Pagilagan and assigned to Du Pont) for reducing the initial yellowness and improving the color stability of a polyamide composition by using a phosphorus compound while preventing the polyamide from increasing significantly in molecular weight during subsequent processing by deactivating the catalytic effect of the phosphorus compound. The Pagilagan disclosures relate to preparing a polyamide composition from an amino acid or a diamine and a diacid, comprising polymerizing at least one polyamide-forming reactant in the presence of, or introducing into a polyamide melt, (a) a phosphorus compound which does not have a direct carbon-phosphorus bond selected from the group consisting of (1) phosphorous acids, (2) phosphorous acid salts selected from the group consisting of phosphorous acid salts of Groups IA and IIA, manganese, zinc, aluminum, ammonia, and alkyl and cycloalkyl amines and diamines, and (3) phosphorous organic esters which undergo hydrolysis in the presence of water to form inorganic phosphorous acids or salts; and (b) a Group IA base selected from hydroxides, oxides, carbonates, bicarbonates, alkoxides, and hydrides.
Similarly, a process is known from U.S. Pat. No. 5,929,200 (R. U. Pagilagan and assigned to Du Pont) for preparing a polyamide composition comprising polymerizing at least one polyamide-forming reactant to form nylon 6,11,12,66,69,610,612 or copolymers thereof, in the presence of, or introducing into a polyamide melt: a phosphorus compound selected from the group consisting of (1) phosphorous acids; (2) phosphorous acid salts selected from the group consisting of phosphorous acid salts of Group IA and IIA, metals, manganese, aluminum, ammonia, and alkyl and cycloalkyl amines and diamines; and (3) phosphorous organic esters which undergo hydrolysis in the presence of water to form inorganic phosphorous acids or salts; and (b) a multivalent metal compound selected from the group consisting of carboxylate and water soluble compounds of Group IIA metals, zinc or aluminum.
It is known that polyamide resins useful in molding and extrusion applications experience undesirable color build-up (i.e., increase in yellowness) on storage and significant molecular weight increases during subsequent melt processing. Similarly, it is known that when polyamide resins are manufactured conventionally without the addition of pigments (e.g., titanium dioxide white pigment) such resins exhibit varying degrees of yellowness in initial color, increasing in yellowness over time. Exposure to high temperatures during subsequent melt processing operations contributes to increased yellowness. During molding and extrusion applications, resins can be subjected to repeated melting, in the form of regrind, which generally results in the molded or extruded resin exhibiting increased yellowness. Therefore, taking into consideration a potential for the prolonged storage time for these resins and the repeated melting during molding and extrusion, there exists a need for improved polyamide resins which initially appear and, over time, maintain a less yellow appearance than known resins.
A known approach to reduced yellowness in polyamides is to use certain phosphorus compound additives. Such phosphorus compounds are color stabilizers for the polyamides and believed to reduce the degree of oxidative and thermal degradation. However, these phosphorus compounds also serve as polymerization (polyamidation) catalysts. Polyamides, containing these phosphorus compounds which act as polymerization catalysts, when subjected to melting temperatures in an extruder or molding machine, undergo rapid polymerization. This polymerization results in an increase in molecular weight as measured by increased relative viscosity (RV). The RV increase is particularly rapid when remelting polyamide resins dried to a condition of low moisture (H2O) content. As a consequence, an increase in molecular weight of the polymer and decreased melt flow of the polyamide is observed. In molding and extrusion applications, such a decrease in melt flow of the polyamide is undesirable.
The known phosphorus compound additives to reduce yellowness in polyamides include phosphorous acids, their salts, and their organic esters. Examples of these phosphorus acids include: hypophosphorus, orthophosphorus, pyrophosphorus and diphosphorus acids.
Furthermore, it is known that this catalytic effect of certain phosphorus compounds on a polyamide polymerization process may be reduced or completely stopped. This deactivation or reduction of the catalytic effects of certain phosphorus compound in the polymer is known to be controllable through the use of certain bases, e.g., sodium bicarbonate, potassium bicarbonate, potassium hydroxide, and sodium hydroxide.
It is also known that this catalytic effect of certain phosphorus compounds on a polyamide polymerization process may be reduced or completely stopped using multivalent metal compounds in the polyamide. This catalyst deactivation effect is obtained from use of multivalent metal salts of phosphorous acid, acetates, stearates and water soluble halides and include the metals: calcium, zinc, barium, manganese, and aluminum.
It is also known that the molecular weight of polyamide material produced, for example, by the polymerization of hexamethylene diamine and adipic acid can be controlled by addition of materials which function as a chain terminator. It is disclosed for example in Jacobs and Zimmerman, “Chapter 12: Preparation of 6,6-Nylon and Related Polyamides” from Polymerization Processes, Edited by Charles Schildknect, John Wiley & Sons, (1977) at Pages 443-444 that a conventional route to molecular weight control during nylon-66 polymerization (no mention of catalyst) is by the addition of monofunctional amines or acids, Such materials are said to act as chain terminators which reduce the average molecular weight of the polyamide produced. Acetic acid is said to be a commonly used chain terminator.
To summarize the known practice, for polyamide polymerization, at least without a phosphorus compound present, use of a chain terminator can act to control molecular weight increase of the polymer produced. When a phosphorus-based anti-yellowing agent is present, prevention of the polymer from increasing in molecular weight during subsequent melt processing has only been shown to be provided by deactivating the catalytic effect of the phosphorus compounds present in the nylon resins. As a result, the amount of phosphorus compound provided to nylon resin is limited by the amount of catalyst deactivation required. Thus, the requirement in the known art to is to include deactivating materials in the polymer. The approach of catalyst deactivation ultimately competes with the beneficial effects of the phosphorus compound to provide initial yellowness and improving the color stability.
While the objective to provide nylon resins used in molding processes with thermal stability and a resistance to yellowing is well-recognized, the means to achieve this objective using phosphorus compounds is also at odds with the moisture content history of the resin. If all resins could be uniformly maintained at a constant moisture content, regardless of storage conditions, there would be no problem of molecular weight build-up in the melt state over time. However, the condition of unchanging moisture content practically never exists for resins in molding and extrusion applications. In general, there exists a variability in the resin moisture levels prior to melting and also for the duration of time in the melted state. Often users of nylon resins in molding and extrusion processes “over-dry” their polymers. The over-dried polymer has a low moisture content which in combination with the catalytic effects of phosphorous compounds, used to maintain high stability and whiteness, drive the molecular weight of the polymer objectionably high. Consequently, a higher than desired, or even a not easily predicted melt viscosity situation exists and molding defects due to melt flow problems result.
There remains in the art an unmet need for a nylon molding resins which can be processed in a predictable and reproducible manner regardless of prior melt history and moisture content. Nylon molding resins having a low yellowness and excellent whiteness stability as a result of phosphorous compound addition and having predictable molecular weight build-up in the melt (or fluid) state are desirable.